Concrete is a composite material that is composed of water, cement, and aggregate. Common aggregates include sand, gravel, or crushed stone. Concrete is a well-known structural component with typical compressive strengths of approximately 2500 psi. More detailed discussions regarding concrete and its properties can be found in Concrete, by S. Mindess and J. F. Young (Prentice Hall, Inc, Englewood Cliffs, N.J. 1981), in Design and Control of Concrete Mixtures, 13.sup.th Ed., by H. Kosmatka and W. C. Panarese (Portland Cement Association, Skokie, Ill., 1988), and in the ACI Manual of Concrete Practice (American Concrete Institute, 1987). There are numerous applications where a lower density concrete may be suitable, useful, or desirable, especially when combined with other attributes such as better processing, higher strength to weight ratio, improved insulation properties, and or enhanced acoustic properties.
Useful concrete compositions of reduced density are produced by the introduction of solid aggregates of lower density, by incorporating significant amounts of air or other gas, or by a combination of these methods such as including expanded polystyrene or other polymer foam. Structural lightweight concrete compositions (85-115 lb/ft.sup.3) are most commonly prepared using lightweight aggregates (35-70 lb/ft.sup.3, as compared to 75-110 lb/ft.sup.3 for normal weight aggregates) such as kiln expanded clays, shales, and slates; sintering grade expanded shales and slates; pelletized or extruded fly-ash; and expanded slags. The use of these lightweight aggregates is limited by their lower limit of density, their availability, phase separation or non-uniformity upon curing, as well the high cost associated with material, fuel, labor, processing, and transportation.
Other types of lightweight aggregates are available. For example, Desalvo, U.S. Pat. No. 4,019,919, used irregular roughened polyethylene particles for some or all of the aggregate, but particles of this nature tend to separate from the mix as well as rise to the surface, leading to non-uniform compositions. Applications of this technology are also limited by polyethylene's density and cost.
Glenn, U.S. Pat. No. 5,595,595, used aquagels as lightweight aggregates. The use of aqueous gels of polysaccharides requires considerable processing that may include heating, cooling, precipitation into immiscible non-aqueous liquids, chemical treatment, filtration, centrifuging, extruding, vibration molding, or forming. Refrigerated storage is required if the aquagels are not used immediately.
Given the inherently low density of gasses and their relative abundance or ease of generation, their incorporation can have significant advantages for lowering the density of concrete. There are two fundamentally different approaches to incorporating air or other low density gasses. One approach generates gas in situ by chemical reaction and the other approach generates small pockets of air or gas either by whipping the concrete or by including preformed bubbles or foam into the wet mix before curing.
The in situ generation of gas typically involves the production of hydrogen gas from the base catalyzed reaction of a finely divided reactive metal species such as aluminum. AEROCRETE.RTM., from Aircrete Corp., and DUROX.RTM., from U.S. Durox Corp. are examples. This approach requires uniform premixing, the fine control of many processing parameters, and significant capital investment for dedicated special equipment. Other drawbacks to this approach may include the use of autoclaves to cure the concrete under pressurized hydrothermal conditions, the need for molds or other types of undesirable processing steps, and limited vertical uniformity (usually less than two feet). Large shapes or complicated designs are usually precluded from this approach.
Although whipping air into concrete is capable of entrapping bubbles and reducing density, this method is not always practical. Foams are often generated separately using surfactants and other foaming agents in combination with water and air before being introduced to a premixed paste of cement, water, and aggregate. Cellulose based foaming agents are described by Kuramoto et al. in U.S. Pat. No. 3,963,507. A commercially available cellulose based foaming concentrate, "CELLUCON", is also available from Romaroda Chemicals Pty., Ltd., Victoria Australia. Bouchard et al. in U.S. Pat. No. 4,373,955 described a hydrolyzed protein based foaming agent and a hydrolyzed protein based foaming concentrate, MEARL.TM., is available from The Mearl Corporation, Roselle Park, N.J.
Previously, economical and convenient surfactant based foams for use in concrete have not maintained their structures for 90 minute mixing cycles as well as prolonged cure cycles. Despite many advantages, concrete compositions including foam aggregates have often been limited to insulating, non-structural, or non-load-bearing bearing applications, since most commercially available foaming agents are not sufficiently stable in cementitious media and the results are often not consistent. In the previous art, the size and distribution of foam cells have been difficult to control and the cells have had a limited period of usefulness or lifetime. The foam cells have tended to agglomerate, coalesce, and recombine to give larger cells and a wide range of sizes. Long mixing times, such as required for transportation from a concrete production facility to a construction site, have been precluded due to bubbles collapsing and air escaping from the mix. Even when additives have been used to stabilize these foams, such as described by Johansson et al. in U.S. Pat. No. 5,160,540, it is not always practical to add the foam to the cementitious mix at the concrete station since the foam's stability and useful lifetime may prevent transportation. Nakano et al. in U.S. Pat. No. 4,683,003 alter the cure speed to match the foam's useful lifetime using accelerators, retarders, and autoclaving. This method is not trivial and it is impractical for many targeted applications.
A colloidal solution, or sol-gel, stabilizer is described by Gelbman et al. in U.S. Pat. No. 4,900,359. This method may extend the useful lifetime of the foamed concrete, but requires a minimum of 5 percent sol-gel in the foam. The cost of raw materials and the difficult preparation and mixing are prohibitive for many applications using this approach.